Not by clicking on what looks like a power button in the Start menu, that's for sure. Clicking that will put the computer into a sleep mode, rather than switch it off. Presumably that's part of Microsoft's plan to make the PC an 'always-on' device, something Bill Gates was loudly trumpeting back when Windows XP was launched. To switch it off, you need to click the arrow, then select Shut Down from the menu.

One of the designers of that part of the operating system vents his spleen on his blog, about how much hassle the team went through just to design that one switch. And over on his blog, software developer Joel Spolsky gets stuck in to the same subject: "I'm sure there's a whole team of UI designers, programmers, and testers who worked very hard on the OFF button in Windows Vista, but seriously, is this the best you could come up with?"